The genome instability seen in many cancers is thought to generate the mutations that drive tumorigenesis (Hanahan and Weinberg 2000). The defects that lead to this instability, however, may be a vulnerability by which cancer cells can be specifically targeted and killed by chemotherapies (D. A. Chan and Giaccia 2011). In the presence of defects causing higher than normal levels of DNA damage, cells may become reliant upon compensating pathways that reduce the impact of the DNA damage and prevent the cells from activating death pathways. If these non-essential compensating mechanisms are now essential in the cancer cells, then treatments inactivating these mechanisms would specifically kill cancer cells in a genotype-specific fashion. This targeting is essentially a therapy-based form of synthetic lethality, which is observed when inactivation of two genes individually has minimal effect, but simultaneous inactivation of both results in decreased cell growth or cell death. The success of poly(ADP-ribose) polymerase (PARP) inhibitors against cancers with defects in the BRCA1 and BRCA2 genes demonstrates that a synthetic lethality inspired approach can be successful in the context of defects in double-strand break (DSB) repair (Bryant et al. 2005; Farmer et al. 2005). Inactivation of PARP is unlikely to be the only mechanism by which BRCA1 and BRCA2 deficient cancers can be targeted. Systematic screening for additional genes or pathways that are required for BRCA1 and BRCA2 deficient cancer cell survival, however, is both expensive and technically challenging due to the requirements for a high-throughput experiment, difficulties in identifying truly lethal conditions, and incomplete inactivation of genes that are knocked down. Here, I will use genetic interactions with mutations in the DSB repair pathway in the yeast Saccharomyces cerevisiae to identify candidate human genes required for the survival of BRCA1 and BRCA2 deficient cancer cell lines. Given the highly conserved nature of the eukaryotic DNA repair mechanisms (Aggarwal and Brosh 2012), I will use previously identified synthetic lethality interactions as well as newly identified interactions giving rise to increased genome instability and increased sensitivity to DNA damaging agents to identify yeast genes of interest. I will then use siRNA knockdown of human homologs of these yeast genes to determine whether equivalent interactions exist in human cancer cell lines. Finally, I will explore if those interactions validated in human cancer cell lines can be used as a target to induce cell death. I expect these experiments will provide a rational approach to identify a highly enriched set of new targets for future therapeutic development.